Thermal fluid sensor with encapsulated fluid region

A fluid sensor for sensing a concentration or composition of a fluid, the sensor comprising: a semiconductor substrate comprising a first etched portion and a second etched portion; a dielectric region located on the semiconductor substrate, wherein the dielectric region comprises a first dielectric membrane located over the first etched portion of the semiconductor substrate, and a second dielectric membrane located over the second etched portion of the semiconductor substrate; two temperature sensing elements on or within the first dielectric membrane and two temperature sensing elements on or within the second dielectric membrane; an output circuit configured to measure a differential signal between the two temperature sensing elements of the first dielectric membrane and the two temperature sensing elements of the second dielectric membrane; wherein the first dielectric membrane is exposed to the fluid and the second dielectric membrane is isolated from the fluid.